FIG. 2 shows a mechanism deck 2 already known for use in such disk recording or playback devices. The deck 2 comprises a chassis 4 formed with an opening 40, a guide rod 41 extending across the opening 40, and a turntable 31 provided at an end portion of the opening 40 for rotating a disk 6. Mounted on the guide rod 41 is a pickup 3 having an object lens 30 and movable toward the turntable 31. The pickup 3 is driven by a motor M1 on the chassis 4 by way of a gear train 32 and worm 33.
Although the voltage to be applied to the motor M1 is variable to alter the drive torque thereof in the following description, the motor M1 may be so adapted that the drive torque thereof can be altered by varying the current to be supplied thereto.
The disk 6 is housed in a cartridge 60, which is provided with a slidable shutter 61. With the cartridge 60 placed on support pins 42 on the chassis 4, the shutter 61 slidingly moves to expose the lower surface of the disk 6, whereupon a beam is projected from the lens 30 of the pickup 3 for recording or playback.
With reference to FIG. 3, the disk 6 is 64 mm in outside diameter and 29 mm in inside diameter. An inner peripheral region ranging from 29 mm to 32 mm in diameter is referred to as a lead-in area A, a region ranging from 32 mm to 61 mm in diameter as a program area B, and an outer peripheral region ranging from 61 mm to 64 mm in diameter as a lead-out area. Recorded in the lead-in area A is a so-called TOC (table of contents) which is a summary of the information recorded on the disk. The pickup 3 moves along a phantom line R1 (see FIG. 3) through the center of the disk and the lens 30 on the pickup 3.
As shown in FIG. 4, the disk of the type described has grooves called pregrooves 62 and formed over the entire area of its rear surface circumferentially thereof except the lead-in area (A, in FIG. 3), the grooves extending slightly in zigzag. An address is provided every distance of movement of the pickup 3 along the zigzag groove for 13.3 msec, and is detectable even when no data is recorded on the disk. The grooves are approximately equidistantly spaced from inside outward. In place of pregrooves 62, pits (not shown) are formed in the lead-in area.
The distance of movement for 13.3 msec is termed one sector, and 36 sectors correspond to the distance of one cluster. The disk has the location of 0 cluster on a phantom circle with a diameter of 32 mm. The smallest cluster is -134 cluster on a phantom circle with a diameter of 29 mm, and the largest cluster is 2060 cluster on a phantom circle with a diameter of 64 mm.
For accurate reading of signals from the disk, it is necessary to activate a focus servo, tracking servo and speed servo. The term the "focus servo" refers to a servo for delicately moving the lens 30 of the pickup 3 upward or downward for focusing a beam from the lens 30 on the rear surface of the disk. The known astigmatism method is mainly used for the servo. The term the "tracking servo" refers to controlling the position of the beam so that the focus of the beam correctly tracks the pits in the disk. The known beam method or push-pull method is used. The term the "speed servo" refers to controlling the rotation of the disk so as to give the disk a constant linear speed, i.e., 1.4 m/sec. The servo is activated based on the difference between a reference signal on the disk and a reference signal on the circuit.
For recording or playback, a beam spot is first projected on the rear surface of the disk from the lens 30 of the pickup 3, and the focus servo is activated to move the lens upward or downward and focus the beam on the disk rear surface. The lens 30 is thereafter moved along the rear surface of the disk, and the tracking servo is activated for the beam to track the recording groove correctly, with the speed servo activated. After address data is read, the lens 30 or the pickup 3 is moved to move the beam to a desired address to read a desired signal.
In the following description, activating the focus servo, tracking servo and speed servo in sequence will be referred to as "turning on the servo," and inactivating the servos as "turning off the servo."
However, the disk recording or playback device of the type described has the following problems.
It is desired that the device be adapted for so-called random access, such that the pickup 3 is quickly movable to a desired position for the start of playback. However, if the pickup 3 is moved at an excessively high speed, the target position is not detectable accurately, for example, because the pickup 3 moves past the position. Further if the speed of movement is excessive, another problem arises, for example, in that the gear train 32 shown in FIG. 2 for transmitting power from the motor M1 to the pickup 3 gives off an increased noise.
The speed of movement of the pickup 3 varies from device to device due, for example, to fluctuations in the drive torque of the motor M1 and the load torque of the gear mechanism, with the result that some pickup 3 are too great in the speed of movement while others are too slow. Such variations become pronounced in random access.
FIG. 11 shows a closed loop proposed in view of the above problems (see JP-A-195045/1996). The closed loop has an intermediate point 90 provided between a motor M1 and a driver circuit 9 for energizing the motor M1. The load 91 due to the mechanical load of the motor M1 and to the counterelectromotive force resulting from the rotation of the motor M1 is converted as at 92 to a voltage, which is applied to the intermediate point 90, whereby a voltage corrected by considering the load 91 is applied to the motor M1 to stabilize the speed of the motor M1. However, the circuit becomes complex in construction because the motor M1 is controlled by the closed loop every revolution thereof.